9.27.3.4 Assembler Directives

LOC

The LOC directive sets the current location to the value of the operand field, which may include changing sections. If the operand is a constant, the section is set to either .data if the value is 0x2000000000000000 or larger, else it is set to .text. Within a section, the current location may only be changed to monotonically higher addresses. A LOC expression must be a previously defined symbol or a “pure” constant.

An example, which sets the label prev to the current location, and updates the current location to eight bytes forward:

prev LOC @+8

When a LOC has a constant as its operand, a symbol __.MMIX.start..text or __.MMIX.start..data is defined depending on the address as mentioned above. Each such symbol is interpreted as special by the linker, locating the section at that address. Note that if multiple files are linked, the first object file with that section will be mapped to that address (not necessarily the file with the LOC definition).

LOCAL

Example:

 LOCAL external_symbol
 LOCAL 42
 .local asymbol

This directive-operation generates a link-time assertion that the operand does not correspond to a global register. The operand is an expression that at link-time resolves to a register symbol or a number. A number is treated as the register having that number. There is one restriction on the use of this directive: the pseudo-directive must be placed in a section with contents, code or data.

IS

The IS directive:

asymbol IS an_expression

sets the symbol ‘asymbol’ to ‘an_expression’. A symbol may not be set more than once using this directive. Local labels may be set using this directive, for example:

5H IS @+4

GREG

This directive reserves a global register, gives it an initial value and optionally gives it a symbolic name. Some examples:

areg GREG
breg GREG data_value
     GREG data_buffer
     .greg creg, another_data_value

The symbolic register name can be used in place of a (non-special) register. If a value isn’t provided, it defaults to zero. Unless the option ‘--no-merge-gregs’ is specified, non-zero registers allocated with this directive may be eliminated by as; another register with the same value used in its place. Any of the instructions ‘CSWAP’, ‘GO’, ‘LDA’, ‘LDBU’, ‘LDB’, ‘LDHT’, ‘LDOU’, ‘LDO’, ‘LDSF’, ‘LDTU’, ‘LDT’, ‘LDUNC’, ‘LDVTS’, ‘LDWU’, ‘LDW’, ‘PREGO’, ‘PRELD’, ‘PREST’, ‘PUSHGO’, ‘STBU’, ‘STB’, ‘STCO’, ‘STHT’, ‘STOU’, ‘STSF’, ‘STTU’, ‘STT’, ‘STUNC’, ‘SYNCD’, ‘SYNCID’, can have a value nearby an initial value in place of its second and third operands. Here, “nearby” is defined as within the range 0…255 from the initial value of such an allocated register.

buffer1 BYTE 0,0,0,0,0
buffer2 BYTE 0,0,0,0,0
 …
 GREG buffer1
 LDOU $42,buffer2

In the example above, the ‘Y’ field of the LDOUI instruction (LDOU with a constant Z) will be replaced with the global register allocated for ‘buffer1’, and the ‘Z’ field will have the value 5, the offset from ‘buffer1’ to ‘buffer2’. The result is equivalent to this code:

buffer1 BYTE 0,0,0,0,0
buffer2 BYTE 0,0,0,0,0
 …
tmpreg GREG buffer1
 LDOU $42,tmpreg,(buffer2-buffer1)

Global registers allocated with this directive are allocated in order higher-to-lower within a file. Other than that, the exact order of register allocation and elimination is undefined. For example, the order is undefined when more than one file with such directives are linked together. With the options ‘-x’ and ‘--linker-allocated-gregs’, ‘GREG’ directives for two-operand cases like the one mentioned above can be omitted. Sufficient global registers will then be allocated by the linker.

BYTE

The ‘BYTE’ directive takes a series of operands separated by a comma. If an operand is a string (see Strings), each character of that string is emitted as a byte. Other operands must be constant expressions without forward references, in the range 0…255. If you need operands having expressions with forward references, use ‘.byte’ (see Byte). An operand can be omitted, defaulting to a zero value.

WYDE

TETRA

OCTA

The directives ‘WYDE’, ‘TETRA’ and ‘OCTA’ emit constants of two, four and eight bytes size respectively. Before anything else happens for the directive, the current location is aligned to the respective constant-size boundary. If a label is defined at the beginning of the line, its value will be that after the alignment. A single operand can be omitted, defaulting to a zero value emitted for the directive. Operands can be expressed as strings (see Strings), in which case each character in the string is emitted as a separate constant of the size indicated by the directive.

PREFIX

The ‘PREFIX’ directive sets a symbol name prefix to be prepended to all symbols (except local symbols, see MMIX-Symbols), that are not prefixed with ‘:’, until the next ‘PREFIX’ directive. Such prefixes accumulate. For example,

 PREFIX a
 PREFIX b
c IS 0

defines a symbol ‘abc’ with the value 0.

BSPEC

ESPEC

A pair of ‘BSPEC’ and ‘ESPEC’ directives delimit a section of special contents (without specified semantics). Example:

 BSPEC 42
 TETRA 1,2,3
 ESPEC

The single operand to ‘BSPEC’ must be number in the range 0…255. The ‘BSPEC’ number 80 is used by the GNU binutils implementation.